Biological treatments, as typified by the activated sludge method, are widely used in the purification of waste water. In the activated sludge method, the biological treatment uses microorganisms and oxygen to immobilize organic polluting substances in the waste water into an activated sludge, which is partly removed by decomposition into water and carbon dioxide (self-digestion).
The presence of dissolved oxygen in the reaction system is essential in biological waste water purification processes, such as the activated sludge method and the like, and often the supply of dissolved oxygen to the reaction system is the rate-determining condition.
Air has been used as an air disperser unit (bubbling) in order to promote biological reactions in the reaction system. The utilization efficiency of oxygen, however, was very low because the size of the gas bubbles introduced by the air disperser unit into the treated water was very large, so that most of the gas rapidly rose up through the treated water to be dissipated from the water surface.
Accordingly, there have been attempted to replace aeration schemes using standard air with aeration schemes using oxygen-enriched air or pure oxygen, to increase thereby the aeration volume. The dissolution efficiency of air or the like in conventional aeration technologies ranges ordinarily from 5 to 6%, and is at best of about 10%.
Upon dissolving a poorly soluble reactive gas such as oxygen gas or the like in a solution, as is known, increase of the pressure results in an increased amount of the gas dissolved. In conventional reaction vessels, as a rule, biological reactions take place under atmospheric pressure, so that the only pressure-contributing factor is the water depth pressure in the reaction vessel (usually, a pressure of about 0.05 MPa for an effective water depth of 5 m).
Increasing the depth of the vessel (tank) has also been attempted as a way of exploiting the water depth pressure of the vessel. Deep shafts reaching depths of 40 to as great as 150 m have been studied in this regard.
In order to bring oxygen gas (air) bubbles to such depths, however, it is necessary to increase to 2 m/sec or greater the flow velocity of the water to be treated that descends into the deep shaft to resurface again, which increases power consumption for aeration; moreover, the scarcity of suitable locations where strata (bedrock) are free of water leak concerns, and the large construction costs that must be incurred for reaction vessel installation have curtailed the widespread use of such deep shafts, despite oxygen gas utilization rates of 90% that can be realized thereby.
Although the pressurized reaction vessel approach has not been ruled out, it has not been adopted because the improved effect of pressurization has limitations as regards cost effectiveness, given the oxygen (air) utilization rates in the current state of the art of aeration technology, and because of the enormous costs involved in providing air-tightness/pressure-resistance to reaction vessels in public sewerage facilities, where the daily inflow can reach from tens to hundreds of thousands of tons, with loadings of about 10 tons/m2 generated by water depth pressures of 0.1 MPa in ordinary reaction vessels.
Ozone is also used as a means for reducing the volume of excess activated sludge in waste water treatment systems employing the activated sludge method (see Japanese Examined Patent Application Publication No. H05-85236, Japanese Patent Application Laid-open No. H06-206088, Japanese Examined Patent Application Publication No. S57-19719).
Even so, gas-phase solubility/dispersibility in liquids are much the same as those of conventional aeration technology.
The present inventor previously invented a waste water purification process by the activated sludge method, in which a gas is dispersed and held in a liquid in the form of (ultra)fine bubbles (see Japanese Patent Application No. 2002-212598).
Herein, dispersing and holding of a gas in a liquid in the form of (ultra)fine bubbles using a stirring device (see Japanese Patent Application Laid-open No. H07-124577 or the like can improve dramatically gas solubility and dispersibility in a liquid, and enables to accomplish a gas-phase utilization of nearly 100%. However, activated sludge reaction systems are operated, as a rule, as a open system under the atmospheric pressure, and necessitate therefore further improvement as regards reducing the amount of excess sludge produced as a result of the waste water cleaning action.
Patent Document 1: Japanese Examined Patent Application Publication No. H05-85236
Patent Document 2: Japanese Patent Application Laid-open No. H06-206088
Patent Document 3: Japanese Examined Patent Application Publication No. S57-19719
Patent Document 4: Japanese Patent Application No. 2002-212598
Patent Document 5: Japanese Patent Application Laid-open No. H07-124577